I've been trying to think of a device as you've described for a while now. It always ended up looking like it'd cost me a few hundred bucks to get all the parts I'd need to put together. If you don't mind sharing a picture at some point I / we'd appreciate it!
It would seem that in an ideal state you could get a bit of both... some CO2 going by to eliminate any oxygen present in whatever chamber they go into, and then after some period of time that flow is redirected end they are kept isolated.
I'd envisioned something that the hops would dump into, some CO2 could be moved by (from a tank or fermentation), and then they could be released into the fermenter itself. They would be stored in the freezer until moments before being needed. Or perhaps allowed to warm up to room temp for an hour, but then dumped.
Time for which to load them is an open question. When the yeast is pitched, at high krausen, at the end of fermentation, or even after a brief chilling period have all been used with success. I won't try to say what's best. Just a few days ago I put some into my fermenter at high krausen and in a few weeks will see how that goes. The choice was that the bubble layer on top would "protect" the beer from any O2 and that the active fermentation would additionally shove it out pretty quickly. The upside, if it works, is that no special devices are needed. Just open, dump, close and walk away.
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